Slowed motor development is a well-known phenomenon among children who suffer varied birth defects and developmental disabilities. Few treatments have been identified that will reliably and significantly enhance the motor and reflex development of disabled babies. However, recent studies indicate that specific rotational vestibular stimulation may produce positive changes in motor and reflex development among delayed infants. Preliminary evidence suggests that some preambulatory children exhibiting Down syndrome, cerebral palsy, and undifferentiated birth defects improve significantly in motor and reflex development following a regimen of semicircular canal stimulation. The purposes of this project are: (a) to study the effects of vestibular stimulation in three groups of babies (normally developing, Down syndrome, and cerebral palsy) with sample sizes sufficient to make inferences about differential responsivity to vestibular stimulation, (b) to study these babies longitudinally in order to separate treatment effects from maturational variables and to determine the long-term effects of the intervention, (c) to measure motor, reflex, physiological and cognitive functions, and (d) to study the relationship between motor development and the stereotyped movements of the babies. 20 Down syndrome, 20 cerebral palsy, and 20 normally developing preambulatory babies between .5 and 2.5 years will serve as subjects in a cross-over longitudinal design that will permit between-groups and within-subjects comparisons. The project will consist of two phases, with half of each subject population in each phase. During the second phase, the subjects will be followed at 3-month intervals to ascertain the long-term effects of the vestibular treatment. Pretest, interim, and posttest measures include motor and reflex scales, an environmental interest inventory, the Bayley Scales, parent questionnaires, and several physiological measures including postrotary nystagmus, heart rate, heart rate variability, respiration, blood pressure, weight and muscle tone. A treatment session of vestibular stimulation consists of rotating the child 10 one-minute periods of rotation in different body positions in a motorized apparatus. Sessions will be conducted 10 times over 2 weeks, on 4 different occasions during the experimental phase.